Aminocyclopyrachlor has provided excellent control of many perennial weed species including leafy spurge, but control of yellow toadflax has been inconsistent. 14C-aminocyclopyrachlor absorption was rapid in both leafy spurge and yellow toadflax and averaged 72% 48 h after treatment (HAT). However, translocation within the plant differed by species. More 14C translocated to the aboveground portion of yellow toadflax (28% of applied) compared to leafy spurge (16.5% of applied). There was rapid translocation of 14C-label to the roots of both species but more reached the belowground portion of leafy spurge than yellow toadflax. Over 12% of applied 14C translocated into leafy spurge roots within 24 HAT but declined to 2% by 192 HAT. In comparison, only 2% of applied 14C was found in yellow toadflax roots 24 HAT, and just 0.15% remained in belowground plant parts by 192 HAT. The inconsistent long-term control of yellow toadflax with aminocyclopyrachlor is likely due to poor translocation to the root system, which would allow for rapid regrowth in this hard to control perennial species.